Laser spike annealing for GaN LEDs

ABSTRACT

Methods of performing laser spike annealing (LSA) in forming gallium nitride (GaN) light-emitting diodes (LEDs) as well as GaN LEDs formed using LSA are disclosed. An exemplary method includes forming atop a substrate a GaN multilayer structure having a n-GaN layer and a p-GaN layer that sandwich an active layer. The method also includes performing LSA by scanning a laser beam over the p-GaN layer. The method further includes forming a transparent conducting layer atop the GaN multilayer structure, and adding a p-contact to the transparent conducting layer and a n-contact to the n-GaN layer. The resultant GaN LEDs have enhanced output power, lower turn-on voltage and reduced series resistance.

FIELD OF THE INVENTION

The present invention relates generally to light-emitting diodes (LEDs),and in particular to the use of laser spike annealing in forming GaNLEDs.

BACKGROUND ART

Gallium nitride (GaN) LEDs have proven useful for a variety of lightingapplications (e.g., full-color displays, traffic lights, etc.), and havepotential for even more applications (e.g., backlighting LCD panels,solid state lighting to replace conventional incandescent lamps andfluorescent lights, etc.) if these LEDs can be made more efficient. Torealize higher efficiency for GaN LEDs, they need to have enhancedoutput power, lower turn-on voltage and reduced series resistance. Theseries resistance in GaN LEDs is closely related to the efficiency ofdopant activation, uniformity of current spreading, and ohmic contactformation.

In GaN, a n-type dopant can be readily achieved using Si and with anactivation concentration as high as 1×10²⁰ cm⁻³. The p-type GaN can beobtained by using Mg as the dopant. The efficiency of Mg doping,however, is quite low due to its high thermal activation energy. At roomtemperature, only a few percent of the incorporated Mg contributes tothe free-hole concentration. Mg doping is further complicated duringMOCVD growth because of hydrogen passivation during the growth process.Hydrogen passivation requires a thermal annealing step to break the Mg—Hbonds and activate the dopant. Typical thermal annealing is performed atabout 700° C. in a N₂ environment. To date, the practical holeconcentration in p-type GaN is still limited to about 5×10¹⁷ cm⁻³. Thislow activation level leads to poor ohmic contact and a large spreadingresistance, which restrict the performance of GaN LEDs.

SUMMARY OF THE INVENTION

An aspect of the invention is a method of forming a GaN LED. The methodincludes forming atop a substrate a GaN multilayer structure having an-GaN layer and a p-GaN layer that sandwich an active layer. The methodalso includes performing laser spike annealing (LSA) by scanning a laserbeam over the p-GaN layer. The method also includes forming atransparent conducting layer atop the GaN multilayer structure. Themethod further includes adding a p-contact to the transparent conductinglayer and a n-contact to the n-GaN layer.

Another aspect of the invention is method of forming a GaN LED. Themethod includes forming a p-contact layer atop a substrate. The methodalso includes forming atop the p-contact a GaN multilayer structurehaving a n-GaN layer and a p-GaN layer that sandwich an active layer,with the p-GaN layer adjacent the p-contact layer. The method alsoincludes forming a n-contact atop the n-GaN layer. The method furtherincludes performing LSA of the n-contact by scanning a laser beam overthe n-contact.

Another aspect of the invention is a GaN LED that includes a substrate,and a GaN multilayer structure formed atop the substrate. The GaNmultilayer structure has a n-GaN layer and a p-GaN layer that sandwichan active layer. The p-GaN layer has been subjected to LSA to have anactivated dopant concentration of greater than about 5×10¹⁷ cm⁻³ and upto about 5×10¹⁸ cm⁻³. The GaN LED includes a transparent conductinglayer atop the GaN multilayer structure, a p-contact formed atop thetransparent conducting layer, and a n-contact formed atop an exposedportion of the n-GaN layer.

Another aspect of the invention is a GaN LED that includes a substrateand a p-contact layer formed atop the substrate. The GaN LED alsoincludes a GaN multilayer structure formed atop the p-contact layer. TheGaN multilayer structure has a n-GaN layer and a p-GaN layer thatsandwich an active layer, with the p-GaN layer adjacent the p-contactlayer. The n-GaN layer has been subjected to LSA to achieve an activedopant concentration of about 3×10¹⁹ to about 3×10²° cm⁻³. A n-contactis formed atop the n-GaN layer.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention, and together with the description serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram of an example structurefor GaN LED;

FIG. 2 is a plot of the annealing temperature T_(A) (° C.) vs. time(milliseconds, ms) and illustrates example annealing temperatureprofiles for three different dwell times of a scanned laser beam whenperforming laser spike annealing (LSA);

FIG. 3 is a close-up side view of a p-GaN layer illustrating the LSAprocess using a scanned laser beam;

FIG. 4 is a schematic view of an example line-type scanned laser beamshape;

FIG. 5 is a schematic diagram of a first example LSA method as appliedto a GaN LED structure formed in the process of creating the GaN LED ofthe present invention such as shown in FIG. 1;

FIG. 6 is similar to FIG. 5 and shows the GaN LED multilayer structureas further including a transparent conducting layer;

FIG. 7 is similar to FIG. 1 and shows the GaN LED being subjected to LSAvia the scanning of a laser beam over the transparent conducting layersurface as well as over the p-contact formed thereon;

FIG. 8 is similar to FIG. 5 and shows an example GaN LED where the GaNLED multilayer structure is reversed so that the n-GaN layer is on topand includes a n-contact, with the GaN LED being subjected to LSA viathe scanning of a laser beam over the surface of n-GaN layer; and

FIG. 9 is plots modeled current (milliamperes, ma) vs. voltage (V)curves that illustrate the performance gains of the GaN LED of thepresent invention (▪) as compared to the prior art performance (♦) asachieved using LSA to lower the series resistance on the operatingvoltage.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made in detail to the present preferred embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same or like reference numbers andsymbols are used throughout the drawings to refer to the same or likeparts. The terms “above” and “below” are relative terms used tofacilitate the description and are not intended as being strictlylimiting.

FIG. 1 is a schematic cross-sectional diagram of an example structurefor gallium nitride (GaN) light-emitting diode (LED) 10. Example GaNLEDs are also described in U.S. Pat. Nos. 6,455,877, 7,259,399 and7,436,001, which patents are incorporated by reference herein. GaN LED10 includes a substrate 20 such as sapphire, SiC, GaN Si, etc. Disposedatop substrate 20 is a GaN multilayer structure 30 that includes an-doped GaN layer (“n-GaN layer”) 40 and a p-doped GaN layer (“p-GaNlayer”) 50 with a surface 52. The n-GaN layer 40 and the p-GaN layer 50sandwich an active layer 60, with n-GaN layer being adjacent substrate20. Active layer 60 comprises, for example, a multiple quantum well(MQW) structure such as undoped GaInN/GaN superlattices. GaN multilayerstructure 30 thus defines a p-n junction. A transparent contact layer(TCL) 70 with a surface 72 resides atop GaN multilayer structure 30. Anexample TCL 70 includes indium tin oxide (ITO). TCL 70 serves to spreadthe current and acts as an antireflection coating to optimize opticaloutput.

GaN LED 10 further includes a notch 80 that exposes a surface portion 42of n-GaN layer 40 that acts as a ledge for supporting a n-contact 90 n.Example n-contact materials include Ti/Au, Ni/Au, Ti/AI, or combinationthereof. A p-contact 90 p is arranged on a portion of TCL surface 72.Example p-contact materials include Ni/Au and Cr/Au.

GaN LED 10 differs from prior art GaN LEDs in at least one of thefollowing ways: a) the dopant activation in p-GaN layer 50 is greater,b) the n-contact 90 n is alloyed using laser spike annealing (LSA), andc) the p-contact 90 p is alloyed using LSA. The methods of processingGaN LED 10 to achieve these differences is described in detail below.

Laser Spike Annealing (LSA)

To increase the activation in p-GaN layer 50, a high annealingtemperature with a short duration is desirable. Using conventional rapidthermal annealing (RTA), the maximum temperature that can be applied islimited by the degradation of the GaN material properties. Onedegradation mechanism is the decomposition of p-GaN layer 50, which isdoped (e.g., with Mg) during an MOCVD growth process. The Mg needs arelatively high annealing temperature for efficient activation, but along duration at high temperature decomposes GaN by nitrogenout-diffusion and reduces the concentration of free-holes in the p-GaN.Typical RTP type annealing processes hold the substrate at 700° C. in anitrogen environment for between several tens of seconds to minutes.

Another degradation mechanism is strain relaxation and dislocationgeneration in p-GaN layer 50. Due to the lattice mismatch, thehetero-epitaxial structure is in a metastable state with built-instrains. Conventional RTA introduces extra strain due to the mismatch inthermal expansion coefficients, and hence accelerates dislocationpropagation and multiplication.

The present invention employs laser spike annealing (LSA), which useshigher temperatures and shorter annealing times than conventionalthermal annealing such as RTA. Example LSA systems suitable for carryingout the methods of the present invention are described in U.S. Pat. Nos.6,747,245, 7,154,066 and 7,399,945, which patents are incorporated byreference herein. Example applications of LSA in the methods of thepresent invention reduce the annealing time by three to four orders ofmagnitude as compared to conventional RTA, enabling higher annealingtemperatures T_(A) (e.g., T_(A)>1,100° C.) without the detrimentalnitrogen-out diffusion and dislocation generation effects.

Enhancing the dopant activation in the doped GaN layer using LSAimproves the contact resistance because the tunneling current is higherand the barrier heights are lower at high dopant concentrations. At highactive dopant concentration, the specific contact resistance ρ_(c)scales as:

$\begin{matrix}{\rho_{c} \propto {\exp\left\lbrack {\frac{4\pi\sqrt{ɛ\; m^{*}}}{h}\frac{\phi_{B} - {\Delta\phi}_{B}}{\sqrt{N}}} \right\rbrack}} & {{Equation}\mspace{14mu} 1}\end{matrix}$where the barrier height change Δφ_(B) is given by:

$\begin{matrix}{{\Delta\phi}_{B} = \left\lbrack {\frac{q^{3}N}{8\pi^{2}ɛ^{3}}\left( {V_{0} - \frac{k_{B}T}{q}} \right)} \right\rbrack^{\frac{1}{4}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

In the above equations, h is the Planck constant, m* is the effectivemass of electron or hole, ε is the dielectric constant of the nitride, Nis the active dopant concentration, q is the elementary charge, k_(B) isthe Boltzmann constant, T is the absolute temperature, and V₀ is thecontact potential.

Increasing the active dopant concentration N increases Δφ_(B), whichreduces the numerator in the exponent of equation 1, and increasing Ndecreases ρ_(c) by increasing the denominator in the exponent ofequation 1. As a result, the contact resistance ρ_(c) decreases withincreasing dopant activation. Example embodiments the methods of thepresent invention increase the activated dopant concentration in p-GaNby a factor of up to about 2.5× (e.g., from about 5×10¹⁷ cm⁻³ to about1.25×10¹⁸ cm⁻³), thereby providing a reduction in total contactresistance (including spreading resistance) of about 60%.

FIG. 2 is a plot of the annealing temperature T_(A) (° C.) vs. time (ms)and illustrates example annealing temperature profiles (curves) forthree different dwell times of a scanned laser beam 120 such as shown inFIG. 3 and FIG. 4. The curves in FIG. 2 represent the annealingtemperature profile of a point P on a surface of a given layer, such assurface 52 of p-GaN layer 50, as shown, as laser beam 120 approaches andpasses over the point. In the calculation, laser beam 120 has a long andthin shape (as taken at a select intensity threshold) at surface 52,e.g., has a length L of about 10 mm and a width W of about 100 μm, or anaspect ratio of about 100:1. Laser beam 120 scans across surface 52 at avelocity V_(S). The dwell time t_(d) is determined by the beam width Wand the scan velocity V_(S). For longer dwell times, thermal conductionpreheats the point P as the laser beam 120 approaches, until the laserbeam strikes the point, thereby bringing the anneal temperature up toits maximum value T_(AM). For shorter dwell times, the thermalconduction is insufficient to pre-heat the silicon and point Pexperiences the maximum annealing temperature T_(AM) for a much shorterduration. This allows for adjusting the annealing temperature profile.

Example LSA Methods for GaN LED Structures

FIG. 5 is a schematic diagram of a first example LSA method as appliedto a GaN LED structure 100 formed in the process of creating GaN LED 10.GaN LED structure 100 includes substrate 20 and GaN multilayer structure30. Scanning laser beam 120 is made incident upon surface 52 of p-GaNlayer 50. Scanning of laser beam 120 is achieved by either scanning thelaser beam or by scanning GaN LED structure 100, e.g., by scanning thewafer (not shown) used in the process of forming GaN LEDs 10. An examplerange for the dwell time t_(d)=W/V_(s) is from about 10 microseconds(μs) to 10 milliseconds (ms). An example range for the maximum annealtemperature T_(AM) is from about 900° C. to about 1,500° C. The maximumanneal temperature T_(AM) is determined by the amount of GaNdisassociation and the lattice mismatch strain relaxation anddislocation in GaN LED structure 100. The depth of the annealing dependson the dwell time and the laser beam intensity. An example laser beamintensity is 400 W/mm². Example GaN multilayer structure 30 has athickness of a few to about 10 μm, and the anneal typically reaches from10 μm to 100 μm, i.e., generally through the GaN multilayer structureand in some cases all the way down to substrate 20. Thus, even thoughincreased dopant activation of p-GaN layer 50 is being pursued, in anexample embodiment there is the additional benefit of increasing thedopant activation in the underlying n-GaN layer 40.

Once the annealing of GaN LED structure 100 is performed, then TCL 70 isapplied atop p-GaN layer surface 52. Notch 80 is then formed, andn-contact 90 n and p-contact 90 p are applied (e.g., deposited) to formGaN LED 10 as shown in FIG. 1

FIG. 6 is similar to FIG. 5 and shows GaN LED structure 100 as furtherincluding TCL 70. An advantage of performing LSA after deposition of TCL70 is that the TCL can serve as a capping layer to prevent nitrogen fromoutgassing during annealing, thereby enabling higher annealingtemperatures T_(A) without material degradation.

FIG. 7 is similar to FIG. 1 and shows GaN LED 10 being subjected to LSAvia the scanning of laser beam 120 over TCL surface 72, including overp-contact 90 p. The relatively low thermal budget of LSA as compared toconventional annealing techniques such as RTA allows for theaforementioned high annealing temperatures to be used without the riskof the metal in p-contact 90 p spiking through the p-n junction.

In an example embodiment of the annealing methods disclosed herein, LSAis used for ohmic alloy formation in p-contact 90 p in the GaN LED ofFIG. 7. Typically, p-type ohmic contact is achieved by alloying Ni/Au attemperatures between 500° C. and 800° C. for 10 to 20 minutes. Highalloying temperatures cause morphology degradation and leakage due toover-diffusion of alloying metal through the p-n junction. Because oflow p-type concentrations, the contact resistance is high, e.g., about1×10⁻³ ohm-cm². This not only causes a large voltage drop but alsogenerates local heating that could degrade the lifetime of the GaN LEDat high current levels. By using LSA, higher annealing temperatures canbe applied without agglomeration. This provides a new opportunity forforming p-contacts 90 p and improving the overall reliability of GaN LED10. In an example embodiment, the p-contact contact resistance is in therange from about 4×10⁻⁴ to about 1×10⁻⁵ ohm-cm². Thus, in an exampleembodiment of the method of the present invention, the combination ofp-contact alloying and increase dopant activation in p-GaN layer 50provides a combined benefit that provides an additional increase in theperformance of the resultant GaN LED 10.

FIG. 8 is similar to FIG. 5 and shows an example vertical GaN LED 10,wherein substrate 20 is metal (e.g., a copper alloy), and GaN multilayerstructure 30 has the n-GaN layer 40 and p-GaN layer 50 reversed fromthat shown in FIG. 5, i.e., the n-GaN layer 40 with a surface 42 isabove active layer 60 and the p-GaN layer 50 is below the active layer.A n-contact 90 n resides atop n-GaN layer surface 42 and a p-contact 90p resides below p-GaN layer and also serves as a reflective layer. Aseparate reflective layer (not shown) may also be added adjacent thep-contact 90 p. GaN LED 10 of FIG. 8 is subjected to LSA via thescanning of laser beam 120 over n-GaN layer surface 42, including overn-contact 90 n. Metal substrate 20 is bonded to GaN multilayer structure30 and has good thermal conductivity that serves to efficientlydissipate heat. Note again that because the annealing reaches down tothe p-GaN level, in an example embodiment this layer also experiences anincreased dopant activation that further enhances the performance of theresultant GaN LED 10.

Establishing ohmic contact of n-contact 90 n to n-GaN layer 40 isusually not a problem due to the generally high dopant concentration inthis layer. Specific contact resistance ρ_(c) below 1×10⁻⁶ ohm-cm² canbe achieved. However, in advanced flip chip LEDs, n-contact formation isperformed after bonding to a different substrate. In this case, thethermal budget (defined as the product of the thermal activationexp{−E_(a)/k_(B)T_(A)} and the annealing duration, where E_(a) is thethermal activation energy, k_(B) is the Boltzmann constant, and T_(A) isthe annealing temperature) needs to be limited to avoid stress anddislocation generation from the mismatch of thermal expansioncoefficient between GaN multilayer structure 30 and (metal) substrate20. In this case, low temperature RTA at 300° C. has been used to formohmic contacts and resulted in a contact resistance ρ_(c)=7×10 ⁻⁴ohm-cm², which is much higher than what is achievable using the higherannealing temperatures and ultra-low thermal budgets associated withLSA. In an example embodiment, a contact resistance ρ_(c) as low as1×10⁻⁶ ohm-cm² is achieved in n-GaN using LSA annealing, leading toimproved GaN LED performance of up to 8% at 350 mA drive current ascompared to LED without laser annealing.

Reducing the contact resistance of the GaN LED leads to improvedperformance. As diode currents increase, the intrinsic resistance givenby (nk_(B)T/qI) (where n is the ideality factor, k_(B) is the Boltzmannconstant, T is the junction temperature, q is the elementary charge, andI is the diode current) decreases to the point that the seriesresistance R_(S) dominates the efficiency of the GaN LED.

FIG. 9 plots modeled current I (milliamperes, mA) vs. voltage (V) curvesthat illustrate the performance gains of GaN LED 10 by using LSA tolower the series resistance on the operating voltage. The plots are forGaN LEDs having different series resistances R_(S), with the “diamonds”curve (♦) modeling conventional GaN LEDs and the “squares” (▪) curvemodeling a GaN LED with 2.5× higher dopant activation in p-GaN using theLSA-based methods of the present invention. Note that the voltage changeΔV is related to the change in the series resistance via therelationship ΔV=IΔR_(S)

At a current I=350 mA, a 40% reduction in series resistance Rs (60% dropin contact resistance) results in about 10% drop in operation voltage Vand hence a 10% increase in LED efficiency in terms of lumens/watt. Amajor part of the series resistance is due to the contact resistance.

The improvements can be even greater for higher drive currentsanticipated being employed by major LED manufacturers in the future. Thetwo curves in FIG. 9 diverge so that at higher driver currents, thevoltage drop is larger. Thus, at a drive current of 700 mA, the GaN LEDformed using the methods of the present invention is anticipated to be15-20% more efficient than a conventionally doped GaN LED. This improvesa GaN LED having a conventional output of 100-lumens/watt GaN LED tohave an output of about 120 lumens/watt.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of forming a gallium nitride (GaN)light-emitting diode (LED), comprising: forming atop a substrate a GaNmultilayer structure having a n-GaN layer and a p-GaN layer thatsandwich an active layer, wherein the p-GaN layer includes dopants boundto hydrogen; performing laser spike annealing (LSA) through at least thep-GaN layer by scanning a laser beam over the p-GaN layer, therebybreaking the hydrogen bonds and activating the dopants therein; forminga transparent conducting layer atop the GaN multilayer structure; andadding a p-contact to the transparent conducting layer and a n-contactto the n-GaN layer.
 2. The method of claim 1, further comprisingperforming the LSA through the transparent conducting layer.
 3. Themethod of claim 2, further comprising performing LSA of the p-contact.4. The method of claim 3, further comprising performing LSA of then-contact.
 5. The method of claim 3, wherein the p-contact has ap-contact resistance, and said performing of LSA of the p-contactresults in the p-contact resistance being in a range from about 4×10⁻⁴ohm-cm² to about 1×10⁻⁵ ohm-cm².
 6. The method of claim 4, furthercomprising: forming a ledge in the GaN multilayer structure andtransparent conducting layer to expose the n-GaN layer; and forming then-contact on the exposed GaN layer.
 7. The method of claim 1, whereinthe LSA has a maximum anneal temperature T_(AM) in the range from about900° C. to about 1,500° C.
 8. The method of claim 7, wherein thescanning of the laser beam is conducted so that the laser beam has adwell time from about 10 μs to about 10 ms.
 9. The method of claim 7,wherein the laser beam has a line-type beam shape with an aspect ratioof about 100:1.
 10. The method of claim 1, wherein the p-GaN layer hasan activated dopant concentration after LSA in the range from about5×10¹⁷ cm⁻³ to about 5×10¹⁸ cm⁻³.
 11. The method of claim 1, furthercomprising forming the active layer to comprise a multiple quantum wellstructure.
 12. The method of claim 1, wherein the dopants consist of Mg.13. The method of claim 1, wherein the transparent contact layerincludes indium tin oxide (ITO).
 14. The method of claim 1, wherein then-contact includes at least one of Ti/Au, Ni/Au or Ti/Al.
 15. The methodof claim 1, wherein the p-contact includes at least one of Ni/Au andCr/Au.
 16. The method of claim 1, wherein the p-GaN layer has a contactresistance, and wherein the total contact resistance is reduced by about60% by performing said laser spike annealing.
 17. The method of claim 1,wherein the laser spike annealing reaches through the GaN multilayerstructure.